1. Field of the Invention
The present invention relates cover of conductor plug that inside trench, and more particularly relates to a method for providing required cover without disadvantages such as porous structure and wastage of conductor plug.
2. Description of the Prior Art
In contemporary semiconductor devices, trench usually is formed in substrate and is used to form shallow isolation by filling dielectric material into trench. However, in some semiconductor devices, such as power integrated circuits, materials filled into trench comprise conductor. Refers to FIG. 1A that briefly illustrates conventional trench structure with conductor plug, wherein conductor plug 11 is located inside trench that is inside substrate 10 and is covered by dielectric plug 12. Moreover, an optional dielectric plug may exist at bottom part of trench and be located under conductor plug 11. It should be noted that substrate 10 can further comprise some structures, such as transistors and conductor lines, in and on it.
Conventional method for covering conductor plug 11 by conductor plug comprises following steps: forms a low O3 flow rate tetraethyl-orthosilicate (TEOS) layer on substrate and fills empty part of the trench; then forms a high O3 flow rate flow rate tetraethyl-orthosilicate layer on the low O3 flow rate TEOS layer; and then etch back these TEOS layers until substrate is not covered by these layers but conductor plug still is covered. Herein, low O3 flow rate TEOS layer is used as a buffer layer and high O3 flow rate flow TEOS layer is used to provide a smoother surface. Moreover, deposit rate of these layers is about 1500 angstroms per minutes and the method can further comprises an anneal process before etch back these layers. However, as FIG. 1B shows, an unavoidable defect of the method is the porous structure of O.sub.3 /TEOS layer 14, especially as is treated by an etch back process, the porous surface will induces some disadvantages in following process.
Beside, when conductor plug 11 is a polysilicon plug, another conventional method for forming dielectric plug 12 is that directly oxidates the polysilicon plug to form oxide plug 15 on conductor plug 11, as shown in FIG. 1C. Advantages of the method include simply process and smooth surface of dielectric plug (oxide plug). However, unavoidable defects include exhaust of polysilicon plug for part of polysilicon plug is depleted during the oxidation process. Further, because the trench structure is part of a semiconductor device, sometime the depleted polysilicon plug is improper and sometime even dielectric plug can not be oxide plug 15.
Accordingly, it is obvious that conventional methods for forming dielectric layer on conductor plug of trench structure are defective and then a mendable method is required, especially when importance of power integrated circuits is increased.